My daughter, who just turned 4, currently has three favorite subjects in the various books we put in her hands. First there are dinosaurs, which she is able to name and roughly classify. Then there is the human body. She knows about cells, muscles, bones and organs. She seems to understand the brain, digestion and the fact that DNA — “It’s ladder-shaped daddy” — carries the code that determines how we look and grow. The latest is space and constellations, planets, the Milky Way and stars. She knows that outer space is weightless and draws her own version of the planets orbiting the sun.